Components of surface acoustic wave type, commonly denoted “SAW” components, according to the acronym corresponding to the English terminology “Surface Acoustic Wave”, are present in numerous applications. For example, they can constitute devices for radiofrequency filtering, notably in telecommunication systems; they can also constitute radiofrequency identification devices or “RFID” according to the acronym denoting the English terminology “RadioFrequency IDentification”, and form identification labels or “tags”; they can also constitute sensors of various physical parameters, such as temperature or pressure. SAW physical sensors can simultaneously ensure the functions of RFID tags and of physical sensors. Hereinafter, reference will be made to a “SAW sensor” to denote either an SAW physical sensor, an RFID tag or a SAW component ensuring these two functions.
SAW sensors present the advantage of being able to be interrogated remotely, but also of not requiring a power supply for such interrogation. Hereinafter, reference will be made to the interrogation of a SAW sensor, to denote equally the gathering of the information relating to its identification and/or to the measurement of a physical parameter. In a typical manner, SAW sensors may be made by depositing metallic patterns on a substrate composed of a piezoelectric material, for example quartz (SiO2), lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). The structures thus formed can in a typical manner comprise a resonator, for example consisting of interdigitated transducers, around which reflectors and electrodes are distributed. Radiofrequency electromagnetic signals can give rise to the generation of an elastic surface wave, e.g. Rayleigh wave, by the transducers, the elastic wave then propagates within the array of electrodes and reflectors, and the transducers then re-emit a radiofrequency signal which may be picked up by an antenna, and analysed by appropriate means. The configuration of these structures makes it possible to influence the propagation of the surface wave and to thus define typical response characteristics of the SAW sensors to determined signals, for example to a pulse of short duration modulated by a radiofrequency signal. A SAW sensor can thus be designed so as to offer its own inherent impulse response, defining its impulse response signature. An exemplary SAW sensor offering good performance, notably in terms of sensitivity to fabrication variations, is that of the reflective delay line, in which a transducer and several groups of reflectors or of electrodes are disposed on the surface of the piezoelectric substrate along the wave propagation axis. The distance between each group of reflectors and the transducer, multiplied by 2, the journey being two-way trip, divided by the phase speed of the wave, corresponds to the delay thus induced. Furthermore, physical parameters such as temperature or pressure may influence the phase speed and the propagation distance between reflector and transducer and therefore modify the delay defined above: more precise analysis of the temporal shape of the signal echoing for example a pulse, thus makes it possible to quantize a physical parameter, provided that the sensitivity coefficient of the delay/quantized value of the physical parameter characteristic is known. When the physical parameter measured is the temperature, this characteristic is for example represented by the temperature coefficient of the propagation delay of the surface waves. In a general way, reference will be made hereinafter to the term physical parameter coefficient of the propagation delay of the surface waves.
A wise choice of the architecture of the SAW sensor and notably of the materials used, for example jointly with the use of several sensors providing a differential measurement, can allow the selective measurement of a given physical parameter, for example temperature, in spite of the simultaneous influence of various physical parameters.
Efficient signal processing strategies have been developed for the purposes of the identification of a single SAW sensor and/or of the gathering of the measurement of the physical parameter, carried out by the latter. On the other hand, the identification and/or the gathering of measurement of a SAW sensor may turn out to be tricky, when several SAW sensors are close to an interrogation device. Indeed, the interrogation device then receives, for example as echo of an impulse signal, a signal which comprises the impulse response signals of the various SAW sensors, the latter possibly being entangled to the point of making it very difficult to discriminate the signals emanating from the various sensors.
Known solutions of the prior art implement complex methods of signal analysis making it possible to dissociate the signals. A known procedure is based on phase coding, and is described in the patent application published under the reference WO 2004/038637. However, such solutions exhibit the drawback of requiring relatively expensive calculation means. According to another known procedure, it is possible to manage code collision by orthogonality. However, according to this procedure, the standards relating to the frequency bands denoted ISM, according to the initials denoting the English terminology “Industrial, Scientific, Medical”, that can be applied preferably to SAW sensors, are not complied with, and it is not permitted to emit signals beyond a certain quantity of energy in the spectral domain.
According to other prior art solutions which are in themselves known, the signatures of the various SAW sensors with which an interrogation device may be confronted, may be defined according to algorithms facilitating their dissociation by signal analysis means. However, such solutions exhibit the drawback of making it more complex to produce the SAW sensors, and of limiting the number of possible signatures according to a given production technology.